Remove assert, add tests

common/array_stack.h

@@ -49,7 +49,6 @@ class ArrayStack {
 
   // Returns the array at a specific index.
   auto PeekArrayAt(int index) const -> llvm::ArrayRef<ValueT> {
-    CARBON_CHECK(index < static_cast<int>(array_offsets_.size()));
     auto ref = llvm::ArrayRef(values_).slice(array_offsets_[index]);
     if (index + 1 < static_cast<int>(array_offsets_.size())) {
       ref = ref.take_front(array_offsets_[index + 1] - array_offsets_[index]);

common/array_stack_test.cpp

@@ -73,5 +73,35 @@ TEST(ArrayStack, Basics) {
   EXPECT_THAT(stack.PeekAllValues(), ElementsAre(5));
 }
 
+TEST(ArrayStack, AppendArray) {
+  ArrayStack<int> stack;
+
+  stack.PushArray();
+  stack.AppendToTop(llvm::ArrayRef<int>());
+  EXPECT_THAT(stack.PeekArray(), IsEmpty());
+  stack.AppendToTop({1, 2});
+  EXPECT_THAT(stack.PeekArray(), ElementsAre(1, 2));
+}
+
+TEST(ArrayStack, PeekArrayAt) {
+  ArrayStack<int> stack;
+
+  // Verify behavior with a single array.
+  stack.PushArray();
+  stack.AppendToTop(1);
+  stack.AppendToTop(2);
+
+  EXPECT_THAT(stack.PeekArrayAt(0), ElementsAre(1, 2));
+
+  // Verify behavior with a couple more arrays.
+  stack.PushArray();
+  stack.PushArray();
+  stack.AppendToTop(3);
+
+  EXPECT_THAT(stack.PeekArrayAt(0), ElementsAre(1, 2));
+  EXPECT_THAT(stack.PeekArrayAt(1), IsEmpty());
+  EXPECT_THAT(stack.PeekArrayAt(2), ElementsAre(3));
+}
+
 }  // namespace
 }  // namespace Carbon::Testing